A surge in sick calls from Air Canada pilots, along with morning fog and a fire at the country’s largest airport, led to snarled travel schedules on one of the busiest weekends of the year.

The inconvenience to thousands of travellers highlights Air Canada’s difficulties establishing peace with its employees as it appeals to Ottawa for assistance once again, this time to address what the carrier claims is an illegal job action by pilots.

Fog and a temporarily closed runway at Toronto’s Pearson International Airport amplified the effects of the pilot shortage during the closing weekend of the hectic March break vacation period. And the carrier is now asking the Canada Industrial Relations Board to declare that the Air Canada Pilots Association authorized strike activity and encouraged members to flout back-to-work legislation, which received royal assent last Thursday.

The union could face fines of up to $100,000 a day if the labour board sides with the airline.

Captain Jean-Marc Bélanger, chairman of the 3,000-member union, said in an internal newsletter to pilots that he conducted a self-assessment and deemed himself unfit to fly, based on his personal workload and sleep deprivation.

“I would have loved to be able to lend a hand as we approach a busy weekend but will be unable to,” Mr. Bélanger wrote on Friday. “I believe in leading by example and I will therefore take the same reduction in income that all of you are faced with when booked off. I had a discussion with my doctor last month and she reminded me of the symptoms that are indicative of a reduced spare mental capacity.”

Air Canada said it has seen a surge in the number of pilots calling in sick since March 8, one day after management tabled its final offer during protracted contract talks. The airline reported 23 flight cancellations across its network on Saturday.

On Sunday at Pearson, there were some 40 Air Canada flight cancellations, though pilots point to the fog and runway shutdown as the main reasons for the Toronto disruptions, which had ripple effects on airports across North America. A fire on Saturday night damaged electrical systems required for signs and lighting on one of five Pearson runways.

On Monday, Pearson's flight schedule showed only a handful of delays and several cancellations, a vast improvement over the weekend's dozens of disruptions.

Air Canada is alleging that some of its pilots have been booking off work in what it asserts is an illegal strike activity sanctioned by the union. More than a dozen pilots cited stress or fatigue for their absence on Saturday in Montreal, for instance.

Mr. Bélanger said many pilots are upset at what they view as harassment from management, including a March 11 letter from Air Canada’s director of labour relations, Harlan Clarke, who warned about disciplinary action for any illegal job action.

“Our in-house specialists tell me that, as in the case of people suffering post-traumatic stress, a period of rest and calm may be enough,” Mr. Bélanger wrote. “The only way to find a way out of this mess is an agreement that is not forced upon us.”

While union leaders fear that the contract dispute will ultimately head to binding arbitration, federal Labour Minister Lisa Raitt said the new fight over booking off sick days needs to be resolved by the Canada Industrial Relations Board. “I understand from Twitter and media that pilots are saying that this is not an illegal work stoppage. But Air Canada is saying there seems to be a pattern,” Ms. Raitt said in an interview Sunday. “So they will go to the CIRB to give evidence and the CIRB will decide if it’s an illegal strike. That’s the appropriate route to take.”

Earlier this month, Air Canada threatened to lock out its pilots while the union representing 8,600 ground crew and mechanics issued a strike notice. But Ms. Raitt referred the two contract disputes to the labour board on March 8, effectively thwarting any work stoppage. As a precautionary measure, she also tabled the back-to-work bill last week.

A senior government official said if the CIRB were to invoke fines, there would be financial hardship. Under the back-to-work legislation, the employer or union faces a fine of up to $100,000 a day for contravening the Protecting Air Service Act. Individuals would face fines of up to $50,000 a day for breaching the act.

In its 15-page application to the labour board, Air Canada argues that “any concerted activity on the part of employees designed to restrict or limit output is a ‘strike’ within the meaning” of the Canada Labour Code.

On top of the labour uncertainty at Air Canada, the airline has been informed that Aveos Fleet Performance Inc., which does aircraft repairs, is getting out of the heavy maintenance business in a move that will result in hundreds of layoffs at Aveos.

Aveos, formerly part of Air Canada, plans to shut airframe maintenance operations in Winnipeg, Vancouver and Montreal, according to the International Association of Machinists and Aerospace Workers.

An Air Canada spokesman said the bulk of the airline’s scheduled heavy maintenance is done by Aveos, but the carrier will be able to secure the services of other companies for aircraft repair and overhaul.